P5 Forces


P5 Contact & Non Contract Forces

A force is a vector quantity and vectors have magnitude and direction. Other vector quantities include: force, velocity, displacement, acceleration, momentum.

Some physical quantities only have a magnitude these are scalar quantities. Other scalar quantities include: speed, distance, mass, temperature, time.

A force is a push or a pull on an object that is coaused by it interacting with something. 

When two objects have to be touching for something to act it is a contact force. These include: friction, air resistance, tensison in ropes, normal contact force

If two object do not need to be touching for the force to act the force is non-contact. These include: magnetic force, gravitational force, electrostatic force.

When two objects interact there is a force on both objects. An interaction pair is a pair of forces that are equal and opposite on two interacting object (the chair pushes on the ground and the ground pushes on the chair equally). The sun's attraction to the Earth and the Earth's attraction to the sun is the same

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P5 Weight, Mass and Gravity

Gravity is a force of attraction between masses. It makes things fall to the ground and gives things a weight

Mass is the amount of "stuff" in an object and does not change. Weight is a force acting on an object due to gravity. Gravity varies with location (it is stronger with larger masses and the closer you are to the mass). Because wiegth depends on the gravitational field strentgh it varies with location.

Weight is a force so it is measured in Newtons (N) with a spring balance. You can think of the force acting on a single point this is the centre of mass. Mass is not a force and it is measured in kilograms with a mass balance

Mass and weight are directly proportional because of the equation:

Weight = Mass (KG) X Gravitational Field Strength (N/KG)

The gravitation field strength on Earth is 9.8 N/KG

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P5 Resultant Forces and Work Done

A resultant force is the overall force on a point or object and is found adding forces going in the dame direction and subtracting any going in the opposite direction. If the resultant force is 0, the object is either stationary or moving at a contsant speed. If two force act in the same direction the resultant force is their sum, if they act in opposite directions the resultant force is their difference.

When a force moves an object through a distance, energy is transfered and work is done on the object, either usefully or it is wasted. The thing producing the force must have a source of energy.

Work done (J) = Force (N) X Distance (m)

1 joule of work is done when a 1 netwon force  causes an object to move 1 metre

Free body diagrams show the forces acting on an object. The size of the arrows shows the magnitude of the object and the direction of the force

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P5 Forces & Elascity

Stretching, compressing and bending requires at least two forces as with one force the object would just move. This transfers energy

An object which has been elastically deformed can go back to its shape when the force is removed. An object has been inelastically deformed if it doesn't return to its shape when the force is removed. When s a spring is compressed energy is transfered to elastic potential energy.

The extention of a spring is directly proportional to the force:

Force (N) = Spring Constant (N/m - measure of stiffness) X Extention (M)

The spring constant is different for different springs - a stiffer spring has a higher constant. The equaltion can be used for compression as well

There is a point at which the force and extention is not proportional. On a graph this is when starts to curve. This point is the limit of proportionality.

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P5 Forces & Elasticity: cont

You can work out the energy stored in a spring as long as it has not passed its limit of proportionality. 

Elastic Potential Energy (J)  = 0.5 X Spring Constant (N/m) X Extention (m)

For elastic deformation this calulates the energy stored and the amount of energy transfered to the spring. This is also the amount of energy that is relased when the spring returns to its normal shape. 

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P5 Distance, displacement, speed and velocity

Distance = scalar. Displacement = vector. Therfore if you walk 5m north then 5m south your diplacement is Om and your distance is 10m.

Speed = how fast you're going (scalar). Velocity = speed in a given direction (vector).

Speed formula   S = VT    Speed (S) = Velocity (V) * Time (T)

Therefore you can have objects traveling at a constant speed with a changing velocity if the direction is changing, for example a car when travelling around a roundabout).

Distance travelled (m) = speed (m/s) X time (s)

Average speeds:  

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P5 Acceleration

Acceleration is how quickly you are speeding up. Decceleration is negative acceleration.  It is measured in m/s^2

Acceleration (m/s^2) = change in velocity (m/s) / time (s)

Uniform acceleration means a constant acceleration - accelereation due to gravity is unifrom acceleration 

Final velocity^2 - initial velocity^2 = 2 X Acceleration (m/s^2) X Distance (m)

Deacceleration is when an oject slows down, it is still the change invelocity per second

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P5 Distance-Time, and Velocity-Time Graphs

Distance time graphs show journeys. Axis: time & distance

Gradient = Speed, flat sections are stationary, straight uphill section mean it is tarvelling at a steady speed, curves mean it is speeding up or slowing down. 

Velocity time graphs also show journeys. Axis: Velcocity & time

Gradient = acceleration, Flat sections are when it travels at a steady speed, uphill is acceleration, downhill is deceleration, curves are changing acceleration.

There will always be friction there to slow things down

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P5 Terminal Velocity

Friction is always there to slow things down this is because if it is moving it will have things rubbing against it. To travel at a steady speed you need to equal the friction force.

Drag (resistance in a fluid) is resistance in a fluid. As speed increases drag increases. To reduce drag the object needs to be streamlined

Parachutes work in the opposite way they increase drag to slow you down.

A falling object reaches terminal velocity when the resultant force is 0, the weight of the object is then equal to the frictional force(air resistance in air, drag in water) on the object. At first the object will accelerate as gravity is more than the frictional force, however as the speed increases so does the friction.

Large, unsteamlined objects have lower terminal velocities. This is because more air resistance acts of bigger surfaces, at any given speed. This means the object will take less time accelerating until the air ressistance is equal to the accelerating force.

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P5 Newton's First and Second Laws

First Law:

If the resultant force is zero the object will move at a steady speed (including if that speed is 0). If the resultant force is not zero the object is: starting, stopping, speeding up, slowing down, or changing direction.

Second Law:

Acceleration is proportional to force. The larger the resultant force the more it accelerates (for the same mass), they are directly proportional. Acceleration is inversely proportional to mass, so an object with a larger mass will have less acceleration for the same force. This is all summed up in this equation:

Force (N) = Mass (kg) X Acceleration (m/s^2)

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P5 I Newton's Third Law

Newton's Third Law:

This says that when two object interact the forces they exert on eachother are equal and opposite. When you push a wall it pushes back with the same force in the oppsoite direction until you stop.

If two ice scaters push against eachother the ligher one will have more acceleration because they have the same force

For it to be Newton's third law the forces must be the same

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P5 Stopping Distances

Stopping distances are important because the longer it takes to stop the more danger the thing you are stopping for (person in the road) is in)

The stopping distance is determined by the thinking distance and the braking distance

The thinking distance is how far the car travels while the driver reacts. This is determined by your speed (travel faster in the time while the driver reacts) and your reaction time. 

The braking distnace is the distance taken to stop when the brakes are applied. It is determined by your speed, the weather (the amount of friction on the road), the condition of the tyres (amount of friction on the tyres), how good the breaks are (how much force they can apply)

The less friction the more time the car will skid for. 

Braking relies on friction, when the brake is pushed the brake pad is pushed on to the wheel, this causes friction while means kinetic energy is transfered to heat and sound. The faster the car is going the more energy the car has so the longer it take. If there is a very large deceleration it can be dangerous as the car can skid.

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Reaction Times & Stopping Distances

Reaction times vary person to person. But it can be affected by tiredness, drugs, or alcohol. Distractions can also affect the time taken 

You can do the ruler drop test to work out recation time using the v^2-u^2 = 2as reaction and the   a = v/t equation

Drivers should leave enough space to stop so if they need to they can do it safely, Speed limits are very important because they affect the stopping distance

Speed affects braking distance more the thinking distance because of the amount of energy you need to tranfer - when the speed is double the energy is 4 times as much ( 2 squared)

30 mph : 9m + 14m

50 mph: 15m + 38m

70 mph: 21m + 75m

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